Coevolution. There was this little inedible snake (let's say a sidewinder, because those are heat seeking species) that snuck up on and entangled birds' wings in the night to exhaust them and finally eat them. The B-Bird was too big for this tactic, so it took the snakes aloft, loosing them after a struggle. This was actually a bonus for the snakes as they thus gained easy access to treetops where a good harvest of nestlings awaited. So the snakes adapted to not obstruct B-bird overly much. B-bird meanwhile benefited because the ruckus caused by the sidewinders dropping into treetops gave them opportunity to ambush some critters that normally kept a low profile. So B-bird adapted by developing the B-plumage, with no hard-to-cling-to plumage in the middle, just bone, skin and sinew, a hard point, if you will.
The docile sidewinder on the hard point still had the innate reflex of orienting towards prospective heat sources, which gave another benefit to the night hunting B-bird.
For reasons of balance, the B-bird often waits for one sidewinder to attach to each wing before taking off. It then homes in on promising targets detected by the snakes, and shrugs them off, diving into the ensuing explosion of activity to find prey.